This invention relates generally to operational amplifiers, and, more specifically, to operational amplifiers having not only a very high gain, but also a very broad bandwidth. The term operational amplifier was originally introduced by workers in the analog computer field to denote an amplifier circuit for performing various mathematical operations, such as integration, differentiation, summation and subtraction. In these circuits, the required response is obtained by the application of negative feedback to a high gain direct current (dc) amplifier, by means of components connected between amplifier input and output terminals in a manner referred to as "operational feedback." The term operational amplifier is now more generally used to designate any high performance amplifier suitable for use with this type of feedback. Operational amplifiers are used not only in analog computation systems, but also in a wide range of other applications in such fields as instrumentation and control system engineering.
On a theoretical level, an ideal operational amplifier has the characteristics of infinite gain between the input and output terminals; infinite input impedance, so that no current flows into the input terminals; zero output impedance, so that the amplifier is unaffected by changes in electrical load coupled to the output terminals, and an infinite bandwidth, i.e. a bandwidth extending from zero to infinity to insure both a response to dc signals and an instantaneous response to change all types of input signal. Another characteristic of the ideal amplifier is that the output voltage signal will be zero when the input signal is zero, regardless of the nature of the input source. This charcteristic is called a zero offset.
In a practical operational amplifier, neither the amplifier gain nor the bandwidth is infinite. One measure of the performance of a practical amplifier is the product of the gain and the bandwidth, which is usually expressed as the frequency at which the gain falls to a value of unity. The best operational amplifiers prior to this invention have unity gain frequencies typically of around one megahertz (MHz), with a few devices providing a unity gain frequency as high as 200 MHz. There is need in some application areas, however, for an amplifier with a unity gain frequency in the range 800 MHz to 1.5 gigahertz (GHz).
The only practical way to provide a very high gain in an amplifier employing bipolar transistors is to provide a large load resistance in the collector circuit of at least one stage of the amplifier. However, simply providing a passive resistance of some hundreds of thousands of ohms in the collector circuit results in unacceptably large voltage drops, and is therefore impractical. The solution employed in operational amplifiers of the prior art is to include an active current source, rather than a passive resistance in the collector circuit. The active current source provides a practically infinite resistance but still conducts a finite current and requires only a small voltage to sustain it. In operational amplifiers, the only practical way of providing such an active current source is by means of a PNP transistor. All operational amplifiers of the prior art have followed this approach, which has two significant drawbacks.
First, in many processes used for the fabrication of monolithic operational amplifiers, i.e. amplifiers formed as a single integrated circuit, it is extremely difficult to control the characteristics of both PNP and NPN transistors simultaneously in the same process. Typically, the NPN devices yield the desired characteristics of high gain and bandwidth, but the PNP transistors tend to have only a moderate gain and an unacceptably low bandwidth. Consequently, the composite amplifier has only moderately good performance characteristics. Secondly, regardless of the fabrication process used, even the most successful operational amplifiers employing PNP transistors to provide a large collector resistance have only moderately high frequency at unity gain, typically in the range of one to two MHz.
It will be appreciated from the foregoing that there is a significant need in the operational amplifier field for an amplifier capable of providing unity gain frequency in excess of one GHz. The present invention fulfills this need.